Electroless or autocatalytic coating of dielectric surfaces is a well-known process finding wide-spread utility in the preparation of such diverse articles as printed circuits, automotive trim, etc.
The normal commercial electroless coating process involves an initial etching of the dielectric substrate by physical or chemical means to improve adherence of the metallic coating. The etching surface is then sensitized by treatment with a solution of stannous ions, e.g., a stannous chloride solution, and then activated by treatment with a solution of palladium chloride. Generally, after etching, most substrates are hydrophillic. Activation is effected by reduction of the palladium ions to the zero valence state by the stannous ions to form palladium metal sites or by formation of a tin/palladium complex on the surface.
In the U.S. Pat. No. 3,011,920, issued Dec. 5, 1961, to Shipley, an alternative process for preparing the substrate prior to electroless coating is described in which the noble metal, e.g., palladium, is applied directly to the dielectric substrate in the form of a colloidal solution containing the metal in the zero valence state. Normally, the metal is formed in the colloidal solution by introducing both palladium chloride and stannous chloride into the solution, the palladium ions being reduced within the solution to the zero valence state by the stannous ions, or by formation of a tin/palladium complex on the surface. In processing an article with this type of catalyst (composed of SnCl.sub.2 and PdCl.sub.2), following a rinse step, an immersion into an accelerator solution is carried out. Typical accelerator solutions are composed of diluted acid such as fluoroboric acid.
Another procedure is described in U.S. Pat. No. 3,783,005, issued Jan. 1, 1974, to Kenney, wherein the sensitizing ions, e.g., stannous ions, are coated onto the substrate first treated by using hydrous oxide colloids of the type described in U.S. Pat. No. 3,657,003, issued Apr. 18, 1972, to Kenney. More specifically, the process described therein comprises forming a hydrous oxide colloid containing stannous ions, either as the hydrous oxide or in combination with the hydrous oxide of another metal, coating the dielectric substrate with the colloid, and thereafter activating the substrate with a solution of palladium or other noble metal ions, the palladium ions being converted to palladium metal by the stannous ions in the colloid. The hydrous oxide colloid used in promoting the stannous ion adsorption to the surface is unchanged in its oxidation state. As an alternative process, Kenney describes activating the surface with a one-step sensitizer-activator solution in which a noble metal ion colloid is applied to the untreated surface and then reduced to the zero valence state.
A similar disclosure of a one-step sensitizer-activator process using palladium is disclosed by F. Pearlstein in Metal Finishing, 53, (8), 59 (1955).
Processes of the above nature utilizing tin/palladium systems have inherent technical and economic disadvantages. Specifically, precious metals in such systems generally exhibit a low overvoltage for hydrogen gas evolution, often resulting in excess hydrogen gas evolution during the electroplating step yielding deposits of poor integrity and porosity which are thus susceptible to chemical attack and poor adhesion. Furthermore, precious metal ions in solution, when brought into contact with metallic surfaces, e.g., copper surfaces, tend to displace the metallic surface and be deposited by a chemical displacement reaction. Such effect is costly and often tends to produce poor adherent conditions. In some cases, too much palladium is deposited, leading to over-catalyzation. See D. Grabbe, "Multilayer board plated-thru-hole Failure Mechanisms," 10th Ann. Proc. Reliability Physics, (IEEE), 1972. Furthermore, such systems cause contamination of the electroless plating baths with palladium, resulting in non-uniform coating and necessitating frequent replacing of the electroless plating baths, high operating costs because of the required use of noble metal ions, and increased costs resulting from the necessity of employing a stabilizer in such baths to inhibit oxidation and hydrolytic reactions of the stannous ions.
The aforesaid disadvantages attendant in the use of palladium or other noble metals are overcome by the process described in copending and commonly assigned U.S. application Ser. No. 521,901, now U.S. Pat. No. 3,993,491 filed Nov. 8, 1974 as a continuation-in-part of U.S. application Ser. No. 422,774, filed Dec. 7, 1973, now abandoned. In the process described therein, the dielectric substrate is treated with stannous and copper ions, either simultaneously or in sequence, the ions then being reduced to a lower oxidation state by treatment with a reducing agent. The substrate is then electrolessly coated by conventional procedures.
Another system utilizing non-precious metal ions is described in U.S. Pat. Nos. 3,772,056 and 3,772,078 to Polichette et al. In the systems described therein, the dielectric substrate to be plated is immersed in a solution of non-precious metal ions, and then dried to promote adherence of the ions to the dielectric surface. The ions may then be reduced to the metallic state by heat or chemical means. A system of this nature is a significant departure from the standard commercial process as a result of the required intermediate drying step to achieve the required adhesion. In standard commercial processes, the article to be plated is passed from one aqueous treating tank to another with intermediate rinsing steps to remove excessive material. A procedure of this nature is essential due to the automation of most plating operations and would require a major modification in standard equipment and procedures, thus rendering the process unattractive and costly. A process for electroless plating, to be commercially feasible must be adaptable to commercial processing techniques.
There is a need for electroless plating compositions and processes overcoming the aforesaid disadvantages while still being compatible with commercial procedures. The present invention provides such compositions and processes.